IP based voice traffic (VoIP traffic) is divided into a talk-spurt in which communication is performed between users, and a silence period in which users are listening without talking. The silence period occupies more than 50% in a conventional call session. Accordingly, various types of voice codec are used to allocate different bandwidths to the talk spurt and the silence period. A represent example of the voice codec includes an adaptive multi-rate (AMR) used in a GSM (Global System for Mobile communication) and a UMTS (Universal Mobile Telecommunications System).
Since voice data are not generated in the silence period, if bandwidths are allocated to the silence period, it can cause a waste of resources. To avoid this, VoIP supports a silence suppression scheme. According to the silence suppression scheme, a vocoder which generates VoIP traffic does not generate any traffic during the silence period, and periodically generates comfort noise to report to its opposite user that call continues to be maintained. For example, a vocoder which uses AMR codec generates a packet per 20 ms during the talk spurt, and generates comfort noise per 160 ms during the silence period.
IEEE 802.16e provides an unsolicited grant service (UGS) scheduling scheme to support a real-time uplink service flow which periodically transmits a data packet of a fixed size, such as T1/E1 or VoIP to which a silence suppression scheme is not applied. According to the UGS scheduling scheme, a base station periodically allocates resources (for example, Data Grant Burst IEs) to a mobile station based on a maximum sustained traffic rate, and periodically transmits data of a fixed size to the mobile station by using the allocated resources.
Furthermore, the IEEE 802.16(e) defines a power saving class (PSC) of a sleep mode to reduce power consumption of the mobile station, which occurs due to the UGS scheduling. In PSC2 which is a kind of PSC, sizes of a sleep window and a listening window are fixed. The sleep window has the same concept as a sleep interval, and the listening window has the same concept as a listening interval. FIG. 1 is a diagram illustrating a concept of UGS scheduling and a concept of power saving class 2 (PSC2) applied to the UGS.
As illustrated in the UGS part, the base station periodically allocates resources of a fixed size to the mobile station, and the mobile station transmits data through the allocated zone. As illustrated in the PSC2 part, the PSC is provided with a listening window and a sleep window, which respectively have a fixed size, to correspond to features of traffic, and transmits data from the listening window through the allocated zone.
Meanwhile, the IEEE 802.16(e) provides a new scheduling scheme called an extended real-time polling service (Extended rtPS) for VoIP traffic which supports a silence suppression scheme. FIG. 2a and FIG. 2b are diagrams illustrating a concept of scheduling of extended rtPS and a concept of PSC2 applied to the extended rtPS (ErtPS).
The base station periodically allocates an uplink bandwidth used for data transmission or bandwidth request, and does not change a size of uplink (UL) allocation until it receives a bandwidth change request from the mobile station. When the mobile station requests bandwidth change, if a bandwidth request size is set to 0, the base station allocates unicast BR opportunity only equivalent to a bandwidth request header (BR header) as illustrated in FIG. 2a, or does not allocate any bandwidth as illustrated in FIG. 2b. 
Referring to FIG. 2a, when the mobile station intends to transmit user data, the mobile station requests a bandwidth request through the bandwidth for transmission of the bandwidth request header. By contrast, if the base station does not allocate any bandwidth as illustrated in FIG. 2b, since there is no chance to request a bandwidth even in case of the presence of user data to be transmitted, the mobile station uses a contention based bandwidth request opportunity or requests bandwidth allocation through a channel (for example, transmission of CQICH codeword), wherein the channel is used when the mobile station periodically transmits control information, such as channel quality, to the base station regardless of transmission of actual user data. However, in case of both FIG. 2a and FIG. 2b, the mobile station repeats the sleep window and the listening window even in the silence period at the same interval and size as those of the talk spurt.
In other words, if comfort noise periodically occurs in the silence period in the extended rtPS (for example, payload of about 7 byte per 160 ms occurs in the AMR), the mobile station transmits the bandwidth request header to the periodically allocated zone (unicast BR opportunity). If PSC is applied to the VoIP service, the size of the sleep window is determined based on unsolicited grant interval (UGI) regardless of the talk spurt or the silence period. Accordingly, the base station periodically (for example, per 20 ms) allocates a bandwidth to the silence period which periodically (for example, per 160 ms in case of AMR codec) generates comfort noise only without data transmission, and the mobile station identifies whether there is downlink traffic per the determined sleep window (for example, 20 ms), or performs a bandwidth request by using allocated resources when there are data to be transmitted. Actually, since noise occurs in the silence period per longer time (for example, 160 ms in case of AMR) than the sleep window, noise occurring per sleep window (for example, 20 ms) of a fixed size can cause unnecessary power consumption.